1. Field of the Invention
This invention relates generally to forced air furnaces, and more particularly to a heat exchanger construction for high efficiency furnaces for central heating systems.
2. Description of the Prior Art
Many gas-fired forced air furnaces for residential use comprise a plurality of clamshell-type heat exchange units, each clamshell unit comprising two formed sheet metal shells welded or otherwise secured together. The units share a ribbon-type burner at the bottom of the heat exchanger to introduce hot combustion products into the inlets of the clamshell units and an exhaust plenum at the top into which the combustion products flow from the outlets of the clamshell units.
U.S. Pat. No. 4,947,548 issued Aug. 14, 1990 to Bentley discloses a dual stage clamshell heat exchanger providing a plurality of condensing heat exchange cells 52 coupled to a lesser number of primary heat exchange cells 32. There may be four condensing heat exchange cells 52 for each primary heat exchange cell 32 (see column 3, lines 29-31), but the coupling boxes 50 do not associate condensing heat exchange cells with individual primary heat exchange cells.
While the clamshell units are comparatively inexpensive, other designs have been pursued as well.
U.S. Pat. No. 3,661,140, issued May 9, 1972 to Raleigh, discloses a furnace using an induced draft blower and a plurality of "heat cells" 40 providing a generally tubular combustion product flow path from the gas burners 48 to the discharge opening 60 into the flue gas collector chamber 77 from which the combustion products are discharged by the "combustion suction fan 78".
More recent patents showing tube-type heat exchangers include U.S. Pat. No. 4,926,840, issued May 22, 1990 to Shellenberger et al, and U.S. Pat. Nos. 5,042,453, issued Aug. 27, 1991 and 4,951,651, issued Aug. 28, 1990, both to Shellenberger. These patents all show heat exchangers comprising a plurality of fire tubes into which gaseous combustion products are directed. A second plurality of tubes smaller in diameter than tubes in the first plurality is coupled to the first plurality of tubes by a manifold. In U.S. Pat. No. 5,042,453, the smaller tubes are shown as being coupled at their outlet ends to a manifold 16 by means of a weldless swedge joint 42.